The present invention generally relates to the field of microwave antennas, and more particularly, to three-dimensional designs for the radiation element of an ultra-wide-band (UWB) monopole antenna with a symmetrical omni-directional radiation pattern for transmitting and/or receiving microwave signals.
UWB generally covers a frequency range between 3.1 GHz and 10.6 GHz. A FCC definition is given e.g. in IEEE 802.15 the disclosure of which is hereby incorporated by reference. According to the IEEE 802.15 Working Group for Wireless Personal Area Networks (see e.g. http://www.ieee802.org/15/) the 802.15 WPAN™ effort focuses on the development of Personal Area Networks or short distance wireless networks. These WPANs address wireless networking of portable and mobile computing devices such as PCs, Personal Digital Assistants (PDAs), peripherals, cell phones, pagers, and consumer electronics; allowing these devices to communicate and interoperate with one another.
The main issues concerning the design of microwave antennas usable for UWB are                to have the capability of a simple planar feeding and a printed low-cost manufacturing,        to achieve a significant cost reduction by simultaneously applying the core substrate of the RF front-end chip as a substrate for the antenna, which means that antenna prints could simultaneously be manufactured by using the layout procedure for classic RF front-end chip circuits, and        to have the capability to cope with symmetrical omni-directional antenna patterns with gains of 0 to 1 dBi (type 1) and/or sector gains of around 6 dBi (type 2).        
Recently, since emphasis has been laid on reducing size, providing increased power efficiency and meeting the requirements of the Federal Communications Commission (FCC) for mobile handset emissions, two additional elements of antenna design have risen in importance that must equally be considered along with conventional design parameters: the enhancement of antenna efficiency and control of the Specific Absorption Rate (SAR).
It is well known that the length of a microwave antenna is inversely proportional to the frequency of transmission: The smaller the antenna size, the lower the antenna efficiency and the narrower is the bandwidth. Thus, as new wireless applications move up in frequency, their antennas correspondingly decrease in size. This natural size reduction, however, is no longer sufficient to meet the demands of consumers. For this reason, antennas are more and more becoming customized components, unique to each wireless manufacturer's performance, size and cost requirements. This evolution is being driven by new radio applications and services which call for antennas that are able                to achieve a higher gain, thereby allowing a reduction in transmitter battery power and a better reception in “dead spots”,        to allow multi-band operation by integrating PCS-based applications operating at 1,900 MHz, applications based on GPS and/or wireless data exchange applications into a single antenna,        to support directional control over handset emissions by allowing more flexible antenna designs which can be used to control the direction of emissions in the vicinity of body tissue and to achieve a better signal reception, and finally        to provide a wider channel bandwidth in order to satisfy the ever-increasing demands for high data rates.        
Usually, microwave antennas are specified according to a set of parameters including operating frequency, gain, voltage standing wave ratio (VSWR), antenna input impedance and bandwidth. If the VSWR is greater than 3, for instance, a matching network has to be placed between the transmitter and its antenna to minimize mismatch loss, although a low VSWR is not a design necessity as long as the antenna is an efficient radiator. Said design is costly and makes an automation of the matching function much slower than designs applying low-power and solid-state tuning elements.
Ultra-wideband (UWB) technology, which was originally developed for ground-penetrating radar (GPR) applications, came into use as a result of researchers' efforts for detecting and locating surface-laid and shallow-buried targets, e.g. anti-personal landmines. With the development of RF electronics the initial desire to discriminate between two closely flying airplanes changed to the quest for constructing a three-dimensional image of a radar target. The potential for direct reduction of the incident pulse duration was soon exhausted and followed by a detailed analysis of target-reflected signals. It became clear that the most important changes in a target response occurred during a transient process with the duration of one or two oscillations. This fact in itself led to the idea of using UWB signals of this duration without energy expenditure for steady oscillation transmission.
Due to the evolution of wireless communications in the area of cellular telephony, wireless local area networks (WLANs) and wireless personal area networks (WPANs), particularly in the frequency range between 0.9 and 5 GHz, higher frequency bands and ultra-wideband wireless communication systems with minimal RF electronics, high data rate performance, low power consumption and a low probability of detection (LPD) signature are urgently needed. Today, UWB system are e.g. used as a wireless RF interface between mobile terminals (cell phones, laptops, PDAs, wireless cameras or MP3 players) with much higher data rates than Bluetooth or IEEE 802.11. A UWB system can further be used as an integrated system for automotive in-car services, e.g. for downloading driving directions from a PDA or laptop for use by a GPS-based on-board navigation system, as an entertainment system or any location-based system, e.g. for downloading audio or video data for passenger entertainment.
Ultra-wideband monopole antennas and modified monopoles are employed in a wide variety of applications today. Traditionally, mobile phones and wireless handsets are equipped with wideband and ultra-wideband monopole antennas. One of the most common λ/4 monopole antennas is the so-called whip antenna, which can operate at a range of frequencies and is capable of dealing with most environmental conditions better than other monopole antennas. However, a monopole antenna also involves a number of drawbacks. Monopole antennas are relatively large in size and protrude from the handset case in an awkward way. The problem with a monopole antenna's obstructive and space-demanding structure complicates any efforts taken to equip a handset with several antennas to enable multi-band operation.
There are a wide variety of methods being investigated to deal with the deficiencies of the common λ/4 monopole antenna, many of these methods being based on microstrip antenna designs. One such promising design is the Inverted-F Antenna (IFA), a distant derivative of the monopole antenna. The IFA utilizes a modified inverted-L low profile structure, which has frequently been used for aerospace applications. The common IFA comprises a rectangular radiation element with an omni-directional radiation pattern and exhibits a reasonably high antenna gain. The bandwidth of the IFA is broad enough for mobile operation, and the antenna is also highly sensitive to both vertically and horizontally polarized radio waves, thus making the IFA ideally suited to mobile applications. Since there is an increasing demand for antennas that can be operated at multiple frequency bands, cellular phone systems nowadays operate at a number of frequency bands (e.g. 900 MHz, 1.8 GHz, and 2.0 GHz).